1. Technical Field
The present application relates to a nitride semiconductor light-emitting element including a non-polar or semi-polar semiconductor layer and also relates to a light source including such a nitride semiconductor light-emitting element.
2. Description of the Related Art
A nitride semiconductor including nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting element, because its bandgap is sufficiently wide. Among other things, nitride semiconductors have been researched particularly extensively. As a result, blue-ray-emitting light-emitting diodes (LEDs), green-ray-emitting LEDs and blue-ray-emitting semiconductor laser diodes, each of which uses a nitride semiconductor, have already been used in actual products.
A nitride semiconductor is represented by the general formula AlxGayInzN (where 0≦x, z<1, 0<y≦1 and x+y+z=1)).
By replacing Ga with Al, the bandgap of a nitride semiconductor can be made larger than that of GaN. By replacing Ga with In, the bandgap of a nitride semiconductor can be made smaller than that of GaN. As a result, not only short-wave light rays such as a blue or green light ray but also long-wave light rays such as an orange or red light ray can be emitted as well. Due to these features, such nitride semiconductor light-emitting elements are expected to be used to image display devices and illumination units, for example.
A nitride semiconductor has a wurtzite crystal structure. FIG. 1 and FIGS. 2A through 2D illustrate the plane orientations of a wurtzite crystal structure which are indicated by four indices (i.e., hexagonal indices). According to the four index notation, a crystal plane or orientation is represented by four primitive vectors a1, a2, a3 and c as shown in FIG. 1. The primitive vector c runs in the direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane” as shown in FIG. 2A. FIGS. 2B through 2D show an m-plane (=(10-10) plane), an a-plane (=(11-20) plane), and an r-plane (=(10-12) plane), respectively. In this description, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar” (a negative direction index) for convenience sake.
FIG. 3 illustrates the crystal structure of a nitride semiconductor according to a ball-stick model. FIG. 4A is a ball-stick model illustrating the arrangement of atoms in the vicinity of the m-plane surface as viewed in the a-axis direction. The m-plane crosses at right angles the paper on which FIG. 4A is drawn. On the other hand, FIG. 4B is a ball-stick model illustrating the arrangement of atoms on the +c-plane surface as viewed in the m-axis direction. The c-plane crosses at right angles the paper on which FIG. 4B is drawn. As can be seen from FIG. 3 and FIG. 4A, N atoms and Ga atoms are present on a plane that is parallel to the m-plane. On the other hand, as can be seen from FIG. 3 and FIG. 4B, a layer in which only Ga atoms are arranged and a layer in which only N atoms are arranged are formed on the c-plane.
In the related art, in fabricating a semiconductor element using nitride based semiconductors, a c-plane substrate, i.e., a substrate of which the principal surface is a (0001) plane, is used as a substrate on which nitride semiconductor crystals will be grown. In that case, due to the arrangement of Ga atoms and the arrangement of N atoms, spontaneous electrical polarization is produced in a nitride semiconductor. That is why the “c-plane” is also called a “polar plane”. As a result, a piezoelectric field is generated in the c-axis direction in the InGaN quantum well that forms the light-emitting layer of a nitride semiconductor light-emitting element. Then, the piezoelectric field generated causes some positional deviation in the distributions of electrons and holes in the light-emitting layer. Consequently, due to the quantum confinement Stark effect of carriers, the internal quantum efficiency of the light-emitting layer decreases. Thus, to minimize the decrease in the internal quantum efficiency of the light-emitting layer, the light-emitting layer to be formed on the (0001) plane is designed to have a thickness of 3 nm or less.
Furthermore, people have recently proposed fabricating a light-emitting element using a substrate, of which the principal surface is either an m-plane or an a-plane (i.e., a so-called “non-polar plane”) or an r-plane or a (11-22) plane (i.e., a so-called “semi-polar plane”). As shown in FIGS. 2A through 2D, in the wurtzite crystal structure, the m-plane is parallel to the c-axis, and consists of six equivalent planes that intersect with the c-plane at right angles. For example, the (10-10) plane which intersects with the [10-10] direction at right angles in FIGS. 2A through 2D is an m-plane. The other m-planes equivalent to the (10-10) plane are a (−1010) plane, a (1-100) plane, a (−1100) plane, a (01-10) plane and a (0-110) plane.
As shown in FIGS. 3 and 4A, on the m-plane, Ga atoms and N atoms are on the same atomic plane, and therefore, no electrical polarization will be produced perpendicularly to the m-plane. That is why if a light-emitting element is fabricated using a semiconductor multilayer structure that has been formed on an m-plane, no piezoelectric field will be produced in the light-emitting layer, thus overcoming the problem that the internal quantum efficiency decreases due to the quantum confinement Stark effect of carriers. This can also be said about an a-plane, which is a non-polar plane other than the m-plane. And the same effects can also be achieved even on a −r-plane or a (11-22) plane (i.e., a so-called “semi-polar plane”).